The aim of this study is to learn more about the regulation of reproductive hormones in adult men, and specifically to determine whether it is testosterone (the main sex hormone produced by the testis) or estradiol (another sex hormone) which is more important in controlling the secretion of lutinizing hormone (LH) and follicle stimulating hormone (FSH) from the pituitary gland. To do this a medication (aromatse inhibitor) will be administered which blocks the body's ability to convert testosterone to estradiol. Our hypothesis is that estradiol has a greater inhibitory effect on LH and FSH secretion than testosterone and that consequently blocking the production of estradiol will result in an increase in LH and FSH levels. Frequent blood sampling will be used in this randomized study to determine the effects on FSH and LH secretion. The broad goal of this protocol is to examine the role of sex steroids in the negative feedback regulation of gonadotropin secretion in the human male. Studies of normal and GnRH-deficient men, employing an aromatase inhibitor to inhibit the conversion of testosterone (T) to estradiol (E2), will permit the effect of selective estrogen ablation on gonadotropin regulation to be elucidated. Such studies are prerequisite to defining not only the pathophysiolgy of male infertility, but also to determine the feasibility of hormonal approches to male contraception. In the present study we sought to overcome the methodological difficulties by using a potent, highly selective aromatase inhibitor, anastrozole, to examine the direct effects of physiologic concentrations of endogenous testosterone on gonadtropin secretion in absence of estrogen.